有没有办法在编译时替换元组元素?
例如,
using a_t = std::tuple<std::string,unsigned>; // start with some n-tuple using b_t = element_replace<a_t,1,double>; // std::tuple<std::string,double> using c_t = element_replace<b_t,char>; // std::tuple<char,double>
解决方法
你可以使用这个:
// the usual helpers (BTW: I wish these would be standardized!!) template< std::size_t... Ns > struct indices { typedef indices< Ns...,sizeof...( Ns ) > next; }; template< std::size_t N > struct make_indices { typedef typename make_indices< N - 1 >::type::next type; }; template<> struct make_indices< 0 > { typedef indices<> type; }; // and Now we use them template< typename Tuple,std::size_t N,typename T,typename Indices = typename make_indices< std::tuple_size< Tuple >::value >::type > struct element_replace; template< typename... Ts,std::size_t... Ns > struct element_replace< std::tuple< Ts... >,N,T,indices< Ns... > > { typedef std::tuple< typename std::conditional< Ns == N,Ts >::type... > type; };
然后使用它:
using a_t = std::tuple<std::string,unsigned>; // start with some n-tuple using b_t = element_replace<a_t,double>::type; // std::tuple<std::string,char>::type; // std::tuple<char,double>
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